With the increasing popularity of the Internet and other content-heavy electronic communication systems, there has been a substantial need for reliable and affordable high bandwidth mediums for facilitating data transmissions between service providers and their customers. In relation to the requirement that such mediums be affordable to consumers, it was determined that the most cost-effective manner for providing service to customers was by using infrastructure already present in most locations. Accordingly, over recent years, the two such mediums most widely meeting these requirements include the cable television (CATV) and the conventional copper wire telephone systems such as a “plain old telephone service” (POTS) or an integrated services digital network (ISDN).
Relating specifically to the adaptation of POTS telephone lines and IDSN lines to carry data at high-bandwidth or ‘broadband’ data rates, a number of Digital Subscriber Line (DSL) standards and protocols have been proposed. DSL essentially operates by formatting signals using various time domain equalization techniques to send packets over copper wire at high data rates. A substandard of conventional DSL is known as Asymmetric Digital Subscriber Line (ADSL) and is considered advantageous for its ability to provide very high data rates in the downstream (i.e., from service provider to the user) direction by sacrificing speed in the upstream direction. Consequently, end user costs are minimized by providing higher speeds in the most commonly used direction. Further, ADSL provides a system that applies signals over a single twisted-wire pair that simultaneously supports conventional POTS or ISDN service as well as high-speed duplex (simultaneous two-way) digital data services.
Two of the proposed standards for ADSL are set forth by the International Telecommunications Union, Telecommunication Standardization Section (ITU-T). A first conventional ADSL standard is described in ITU-T Recommendation G.992.1—“Asymmetric Digital Subscriber Line (ADSL) Transceivers”, the body of which is incorporated herein by reference. A second, more recently proposed standard is the G.992.2 or ‘G.lite’ standard, further described in ITU-T Recommendation G.992.2—“Splitterless Asymmetric Digital Subscriber Line (ADSL) Transceivers”, also bodily incorporated by reference herein. The G.lite standard is a variant of the G.992.1 standard, with modifications directed primarily to work in a splitterless environment (i.e., without a splitter at the remote user end to separate the voice traffic from the digital data traffic).
Prior to any transmission of actual data between the central office ADSL transceiver unit (ATU-C) and the remote ADSL transceiver unit (ATU-R), the two entities must first undergo an initialization procedure designed to familiarize the two entities with each other, identify the bandwidth capabilities for the current session, and further facilitate the establishment of a valid connection. Pursuant to ADSL standards provided by the International Telecommunication Union—Telecommunication Standardization Sector (ITU-T), these initialization procedures comprise the following: 1) a handshake procedure; 2) a transceiver training session; 3) a channel analysis session; 4) an exchange session; and finally 5) an actual data transmission session commonly referred to as “showtime.”
Specifics of the handshake procedure are set forth in ITU-T Recommendation G.994.1—“Handshake Procedures for Digital Subscriber Line (DSL) Transceivers”, the body of which is incorporated by reference herein. The handshake procedure is designed to enable peer components to initiate a communications session between each other and generally includes the exchange of several specific types of messages having predetermined formats. Examples of such messages include the following: capabilities list and capabilities list request messages; mode select and mode request messages; various acknowledge and negative acknowledge messages, etc. Each of the above messages is generally formulated by a protocol processor responsible for ensuring compliance with the requirements for protocol communication.
Because the various ITU-T recommendations identified above are designed to provide guidance to ADSL developers in various geographic locations, different circumstance may exists which impact the method with which the general recommendations are implemented. Accordingly, Annexes to the recommendations have been created that specifically itemize the effect of particular scenarios upon the adoption of the general recommendations. For example, due to noise and other interference generated by these ISDN systems, as well as the potential adverse impact ADSL deployment may have on these existing systems, relatively severe performance limitations have been placed upon ADSL implementation in these regions. Of particular interest in the present application is the effect of a large network of conventional TCM-ISDN (Time Compression Multiplex ISDN) telephone lines on ADSL development. Annex C of the G.992.1 Recommendation directly relates to such circumstances.
As is understood in the art, the data stream of TCM-ISDN is transmitted in one or more TCM-ISDN Timing Reference (TTR) periods. In such systems, the CO transmits data streams in the first half of the TTR period and the CPE (customer premise equipment) transmits data streams in the second half of the TTR period. Accordingly, for the corresponding ADSL system, the ATU-C typically receives NEXT (near-end cross talk) noise from the ISDN in the first half of the TTR period and FEXT (far-end cross talk) noise from the ISDN in the second half of the TTR period. Conversely, ATU-R typically receives FEXT noise from the ISDN in the first half of the TTR period and NEXT noise from the ISDN in the second half of the TTR period. In order to compensate for the effects of the NEXT and FEXT noise, the ATU-C often estimates the FEXTR (FEXT noise at receiver) and NEXTR (NEXT noise at receiver) duration at ATU-R, and the ATU-R estimates FEXTC (FEXT noise at CO) and NEXTC (NEXT noise at CO) duration at ATU-C, considering propagation delay of the subscriber line. Thereafter, the ATU-C transmits symbols by synchronizing with the TTRC and the ATU-R transmits symbols by synchronizing with the TTRR which is generated based upon the received TTRC. FIG. 1 illustrates a conventional timing model for ISDN/ADSL systems.
Due the differing effects of NEXT and FEXT on ADSL transmissions, Annex C of the G.992.1 Recommendation suggests implementing a Dual Bitmapped (DBM) process for framing data prior to transmission using a discrete multitone (DMT) transmission process. In this manner, symbols are created differently depending upon whether they are transmitted during a NEXT period or a FEXT period, with the ATU-C transmitting FEXTR symbols using a Bitmap-FR (in FEXTR duration), and transmitting NEXTR symbols using Bitmap-NR (in NEXTR duration) according to the result of initialization. Similarly, the ATU-R transmits FEXTC symbols using Bitmap-FC (in FEXTC duration), and transmits NEXTC symbols using Bitmap-NC (in NEXTC duration) in the same manner. In accordance with Annex C, the FEXTR/C transmission includes 128 symbols, while the NEXTR/C transmission includes 217 symbols, resulting in a combined hyperframe of 345 DMT symbols.
As a means of controlling symbol transmission, Annex C also affords the ATU-C the capability to disable Bitmap-NC and Bitmap-NR, thereby disabling the transmission of anything but a pilot tone during the NEXT TTR periods. This mode of transmission is conventionally referred to as FBM (FEXT Bitmapped) transmission. The FBM mode uses the DBM technique to transmit data only during FEXT intervals. Accordingly, the ATU-C transmits only the pilot tone during the NEXTR symbol. Consequently, the ATU-R disables Bitmap-NC and does not transmit any signal during the NEXTC symbol. The ATU-C selects the DBM or FBM mode during G.994.1 handshaking using a “DBM” bit.
Another scenario of interest in the present application is that discussed in Annex A to the G.992.1 Recommendation, requirements for ADSL systems operating in a frequency band above the POTS frequency band. As is understood, in order to avoid interference with existing POTS systems, shifts in the ADSL signal Power Spectral Density (PSD) must be made in particular frequency ranges.
Although the recommendations present in the various specification identified above have been implemented to avoid conflict and interference with existing systems, there remains a need in the art of ADSL systems for methods and systems for improving both performance and range of such systems without adversely effecting existing systems, thereby improving upon the recommendations described forth above.
Additionally, there is a need in the art of ADSL systems, for improved systems that maintain compatibility with existing ADSL systems and equipment and which may be implemented with minimal changes to both equipment specification and technical recommendations.